I agree with artist Trina Robbins that you have to have started drawing when you were very young to become good at it later on. But in order to be really fluid, you have to practice a lot and I believe the key is to draw day in and day out.
It's like a batting average at baseball, by doing this you can bring your stats up. Instead of doing one nice drawing out of every five to ten you try if you just draw once in a while, you can bring it up to two nice drawings out of every three if you draw every day.
I have written a lot of jokes for the Zanto comic strip in advance, and I remember that I sort of wrote myself in a corner and had trouble drawing this particular strip because I had stopped drawing for a few weeks.
To get it done, I sat down and put on a silent movie of you-know-who in the DVD player and did a few sketches from that, something I never do. The last drawing I drew of Zara (in the last panel) was a bit off, but it reminded me of Italian artist Guido Crepax, so I intentionally modified the word ballon to look like something Guido Crepax would have done. Guido Crepax' creation Valentina in the 1970s was based on the same silent screen actress that inspired Zara, so it is an homage.
The many monsters I have drawn for monsteraday.com have helped improve my drawing a lot, if only to get me to draw more regularly